Earlier light-emitting diodes (LEDs) are mostly monochromatic LEDs. The light emission of an LED is due to electrons falling to a lower energy level. If a proper energy difference is not available, an ideal LED light source cannot be produced. Thus, earlier technology requires red, blue, and green chips to be assembled together to form a single white LED component. For this reason, the earlier white LEDs are also called multi-chip white LEDs.
Since Nichia Corporation disclosed a phosphor-LED device, which is a light-emitting diode covered with a layer of phosphor or fluorescent powder, more and more applications of LEDs has been developed. For example, a blue LED chip emits blue light passing through a material containing phosphors which fluoresce in yellow; the original blue light and the fluoresced yellow light recombine to form a blended light beam. The color vision of human eyes is based on tree kinds of cone cells, which respectively correspond to perception of red, green, and blue light. Because the yellow fluorescence can stimulate and activate the cone cells associated with red and green light, and the original blue light can stimulate the cone cell associated with blue light, white light can be perceived by the human eyes.
The white light, which is produced by a blue LED chip in cooperation with yellow YAG (Yttrium Aluminum Garnet) phosphor or in cooperation with red and green phosphors, is commonly used in the industry today. Since this type of light source, especially the blue LED chip with the YAG, has a lower cost and the circuit design is simple, it is applied extensively. However, the color rendering of this light source is not good. Some researchers hold an opinion that the future development trend of white LEDs is towards the UV LED (ultraviolet LED) cooperated with red, green, and blue phosphors. Compared with incandescent light bulbs, white LEDs have the advantages of high conversion efficiency, short activation time, long service life and the like. Thus, the phosphor-LED device has been widely used in artificial lighting.
On the other end, monochromatic light sources can serve as an illumination device applied in many fields, such as criminal identification, counterfeit identification, metal aging test, transgenic detection, and fundus examination. Those identification equipment employ monochromatic light as excitation light or illumination light, which can be used to detect physiological fluids, such as blood or semen, or directed at a banknote to reveal the security features thereon, or directed at a metal material to detect the fluorescent coating which has not been erased in the aging lines of the metallic material, or directed at an organism to detect the presence of a fluorescent protein to confirm whether a transgene is successfully introduced to the organism or not.
Most of the illumination devices of the aforementioned identification equipment are required to provide white light illumination. For example, in a forensic examination, investigators should rely on white light image records to conduct cross comparison to confirm the relationship between evidence and the environment to persuade judges or juries of the truth of the evidence. In addition, some identification equipment employs UV light as an excitation light, which cannot be directly observed with human eyes; however, for conducting a proper operation of the equipment, investigators should rely on white light illumination. The applicant of this application has developed some light sources, in which various light emitting elements are employed. Among those light sources, a composite light source may be a combination of a UV LED and a white LED, or a combination of a blue LED and a white LED, or a combination of an infrared LED and a phosphor LED.
For criminal identification, metallic material test, and biological research, it is important that the color distribution and sharpness of an image is not allowed to have little deviation. Particularly, in the case of the image record formed by weak fluorescence, unwanted light should be reduced, and a suitable light source should be selected so that direct reflection from an object to be tested can be reduced to prevent the light beams of direct reflection from entering an imaging device to interfere with an image record. Generally, a low-angle annular light source can be employed to achieve the effect of dark-field illumination and to create scattering to prevent the image of the object from interference.
FIG. 14 shows a conventional device, wherein a light source thereof is in the form of a ring. As shown, the illuminating device 9 includes a cylinder 91, which can be connected with a camera 8 by a battery block. The cylinder 91 is provided with a first light-emitting unit 911 and a second light-emitting unit 913. The first light-emitting unit 911 includes a plurality of high-frequency LEDs 9111. The second light-emitting unit 913 includes a plurality of white LEDs 9131 each containing a yellow phosphor coating. A user can select one of the light-emitting units to be turned on according to an application.
In the conventional device, when the high-frequency LEDs 9111 are enabled to emit UV light, since the white LEDs 9131 are within the illuminating range of the high-frequency LEDs 9111 or on the light-emitting axis or path of the high-frequency LEDs 9111, the yellow phosphor coatings of the white LEDs 9131 can absorb part of the UV light to release yellow fluorescence, which may affect the image of a target under the UV light. Thus, the signal/noise ratio can be reduced significantly, and thus the image may be misinterpreted. Particularly, when the yellow fluorescence released by the target is similar to the yellow fluorescence emitted by the white LEDs 9131, the image formed by the yellow fluorescence is totally covered by the yellow fluorescence emitted by the white LED. This kind of light pollution is due to face-to-face illumination, which may cause an optical capture device to fail in receiving an image, and thus is referred to as “face-to-face glare”.
Someone may try to restore a polluted image to a normal one by using white balance as commonly used in cameras. However, in criminal investigation or biological research, such substantial adjustment for the contents of an image may change the image data itself. Thus, in court offence and defense or in research publication, the credibility of the image data may be challenged, thus leading to an adverse consequence.
In view of the foregoing, there is a need to provide a low-glare fluorescent-powder LED light device, which can be cooperated with an imaging device and does not produce fluorescence when the phosphor or fluorescent powder thereof is illuminated by a high-frequency LED, so that the signal/noise ratio can be increased, and true images can be obtained.